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Examination of defects and long term crack growth in rolled bars during long term storage
KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering.
2014 (English)Independent thesis Advanced level (degree of Master (Two Years)), 20 credits / 30 HE creditsStudent thesis
Abstract [en]

This work has had the purpose to examine defects and the cause of crack growth during long term storage in rolled bars. The bars in question are rolled and stored at Ovako in Hofors. The problem is approached by theoretically examining the possibilities of hydrogen damage, room temperature creep and naturally occurring porosity.

Seven bars of three steel type groups and with different rolling dates were selected and examined for defects with ultrasonic testing. Four of the bars show defects. Specimens containing the defects are cut out and grinded so that the defects can be examined in the scanning electron microscope and analyzed with energy dispersive x-ray spectroscopy.

One of the specimens shows some form of cavity containing iron carbide. One of the others shows a surface with inclusions. The remaining two both shows inclusions and cracks that seem to originate from inclusions.

The theoretical studies show that there are two types of hydrogen damage that could be in effect in the studied case and also that room temperature creep could facilitate but not cause crack growth and that porosity could serve as crack initiation points. Steps should be taken to minimize the problems mentioned above when they are suspected causes. Further examination of inclusions in different steel types should be made if considered a problem.

Abstract [sv]

Detta arbete har haft syftet att undersöka defekter och orsaken till spricktillväxt under långtidsförvaring av spårvalsade stänger. Stängerna i undersökningen är valsade och lagrade hos Ovako i Hofors. Problemet är angripet genom att teoretiskt undersöka risken för väteskador, kryp i rumstemperatur och naturligt förekommande porositet.

Sju stänger indelade i tre stålsortsgrupper och med olika valsningsdatum valdes ut och undersöktes med ultraljud. Fyra av stängerna visade på defekter. Prov innehållande dessa defekter blev utkapade och slipade så att defekterna kan bli undersökta i svepelektronmikroskop och analyserade med energidispersiv röntgenanalys.

Ett av proverna visar på ett hålrum innehållande en järnkarbid. Ett av de andra proverna visar på en yta med inneslutningar. De två återstående proverna visar på inneslutningar och sprickor som verkar utgå från inneslutningar.

De teoretiska studierna visar att det finns två typer av väteskador som är aktuella i det studerade fallet och även att kryp vid rumstemperatur kan underlätta spricktillväxt men inte orsaka den och att porositet kan tjäna som sprickinitieringsställen. Åtgärder för att minimera de nämnda problemen bör tas i fall där de är misstänkta orsaker. Ytterligare undersökningar av inneslutningsbilden bör göras om inneslutningar anses vara ett problem.

Place, publisher, year, edition, pages
2014. , 47 p.
Keyword [en]
Rolled bars, cracking, hydrogen damage, long term storage
National Category
Metallurgy and Metallic Materials
Identifiers
URN: urn:nbn:se:kth:diva-148106OAI: oai:DiVA.org:kth-148106DiVA: diva2:735217
Educational program
Master of Science - Materials Science and Engineering
Supervisors
Examiners
Available from: 2014-08-18 Created: 2014-07-24 Last updated: 2014-08-18Bibliographically approved

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